As a data communication network becomes more and more popular today, a so-called home network where electrical home appliances, computers, and other peripheral units are networked for communication among them is becoming widely used in the home. A home network, which allows communication among networked devices to share the data processing function and to transfer contents among them, provides the user with convenience and comfort. It is expected that a home network will become more widely used in future.
One of the protocols designed for the configuration of such a home network is Universal Plug and Play (UPnP: Universal Plug and Play). Universal Plug and Play (UPnP) allows the user to easily build a network without complicated operations and allows the user on a networked device to receive the services provided by the connected devices without difficult operations and settings. Another advantages with UPnP is that it does not depend on the OS (Operating System) stored in a device and that devices can be added easily.
Under UPnP, a definition file conforming to XML (extensible Markup Language) is exchanged among devices for mutual recognition. The overview of UPnP is as follows:
(1) Addressing processing for acquiring its own device ID such as an IP address
(2) Discovery processing for searching for the devices in the network and, upon receiving responses from the devices, acquiring information, such as the device type and the function, included in the responses.
(3) Service request processing for requesting a device to provide a service based on the information acquired by the discovery processing
By executing the processing procedure described above, the services can be provided and received by devices connected to the network. A device newly connected to a network can perform the addressing processing described above to acquire the device ID, perform the discovery processing to acquire information on other devices connected to the network and, based on the acquired information, issue a service request to the other devices.
For example, when a client wants to play back a content, such as music data and image data stored in a server, on the client side device, the client side sends an acquisition request to the server to request it to send various types of content attribute information stored in the server, such as the title or the artist name of a tune or a movie, data compression mode information (ATRAC: adaptive transform acoustic coding, MPEG: moving picture experts group, etc.) and, as necessary, copyright information. This attribute information is called metadata or meta information.
In response to the request from the client, the server sends metadata (attribute information) on the content, stored in the server, to the client. Based on the metadata acquired from the server, the client displays the content information on the display of the client device according to the predetermined display program. For example, the tune list composed of artist names and titles is displayed on the display. Based on the display information, the user checks or selects a content to be played back and sends a content transmission request to the server. The server receives the content request from the client and, according to the received request, sends the content to the client, and the client side plays back the received content.
In this way, a content stored in the server in the home network can be accessed from other devices connected to the network. For example, a device connected based on the UPnP connection that is a simple device connection configuration described above can acquire a content. When the content is movie data or music data, a TV set or a player, if used as a network connection device, enables the user to watch a movie or listen to music.
However, according to the specification defined by current UPnP, a server that stores contents therein and supplies the contents to the users (clients) has a configuration in which the server assigns at its discretion an identifier (ID) to a content stored in its own storage unit and manages the assigned ID.
When contents are managed in this way, different identifiers (IDs) are assigned to stored contents according to the storage servers even if the contents are the same. For example, when a content has been moved from server A to server B that is a different server, the problem is that, even if the client memorizes the ID corresponding to the content acquired from server A, the user cannot acquire the same content from server B based on the memorized ID.
Integrated management of contents in one fixed server in a home network configuration, where many devices can be connected, would generate a storage capacity problem or a processing load problem. Therefore, the ability to move contents is vital in building a flexible network system. In such a case, if different identifiers are assigned to a content that is moved from one server to another, the efficiency of content management by the user (client) would be decreased.
When a content is recorded on a plurality of recording media in a home server, a configuration in which contents are managed by using different identifiers based on information unique to each recording medium is described, for example, in Patent Document 1 (Japanese Patent Application Publication No. JP2002-237169).
In the configuration described in Patent Document 1, an ID such as “Recording media ID+Content ID” is set as the management ID. Before a content is copied, a copy of the content is created and one record is added to the database. To execute checkout processing in which the entity of the content is moved from an HDD to an optical disc, the recording media ID is set to an optical disc and the content ID is also changed as necessary. To execute check-in processing that is the reverse of check-out processing, the identifier is changed. To execute export processing in which the content is written to an optical disc for movement to another database, the record is deleted from the database. To execute import processing in which an exported content is registered in a database, the recording media ID and the content ID are newly set and other additional information is data read from the optical disc. The ID management configuration like this is described in that document.
As described above, the devices in the conventional configuration such as a server in which contents are stored, the identifiers (IDs) corresponding to contents are individually set and the contents are usually managed based on the IDs that are set. Such a configuration requires the user to change the identifier as the content is moved as described above, and this requirement decreases the content management efficiency of the client.